Bridging an amplifier

I recently watched a YouTube video, a few years old, that featured Flemming Rasmussen, the highly regarded founder and engineer for Gryphon (now retired I believe). Flemming was speaking of his class A monoblock amplifiers in the Antileon Evo, Colloseum and Mephisto. Flemming was expressing his disdain for Monoblocks that are actually 2 internally bridged amplifiers, (such as DNA 500,Clayton M300s and also class A Luxman that can be used as Monoblocks in the BTL mode (bridged tied load), calling these types of Monoblocks inferior and not true Monoblocks. He claims dedicated Monoblocks where all push pull transistors are paralleled, are vastly superior to summing the two channels via bridging (antiphase summing). I’d love to know if most audiophiles and engineers agree with Flemming. I have owned and found both the DNA Monoblocks and Clayton M300 Monoblocks to sound excellent, and a friend uses two M-800A Luxman class A amplifiers in bridged tied load mode (BTL switch) with great clarity and power. Thank you for your thoughts!
Well as Austin Powers put it, "Everyone likes their own brand."

On Wall St its called talking your own book.

A psychologist would probably see it as evidence of cognitive dissonance.

Me, I just see it as way too simplistic. You hear a guy talking like that, you just know he's taking something in which a zillion different aspects interact and boiling it down to the one thing. Which you know is nonsense. Which is why Austin Powers pee joke isn't irreverent, but accurate and very much to the point.
Hmmm, from what I understand, the concept of using a stereo amp in bridged-mono mode is really no different than a true monoblock. They both use "antiphase summing" where one amp board pushes while the other amp board pulls. They both have to use push-pull transistors arrays where one set of transistors is pushing the positive side of the waveform and the other set pulls for the negative side of the waveform. However, stereo amps cannot normally support low impedance loads. For example, DNA 500 is a stereo amp that supports 500 watts into 8 ohms or 900 watts into 4 ohms. When bridged, I suspect that you do not want to run a load under 8 ohms.

The Luxman is designed with higher current, but again the impedance capability is halved when bridged. However, they support 1 ohm loads in stereo and 2 ohm loads bridged.

A dedicated monoblock usually has a power supply design that can adequately support low impedance loads (such as 4 ohm or lower). Based on what I can find, that Clayton M300 is a true monoblock.
Hi millercarbon & auxinput 

Thanks for your input. I guess I’m still not fully understanding the difference between a ‘true’ mono-block vs bridged monoblock....maybe I’m getting stuck on terminology. I thought it was more than a robust power supply? BTW, the Clayton M300s have been described as a bridged Balanced Class A design with essentially two M100s internally bridged in each monoblock....guess I need to read a little more about paralleled vs bridged amplifier designs.
After some research I find that some excellent manufacturers that design dedicated internally bridged Monoblocks such as McCormack DNA 500 and Clayton M300/S2000 can build designs that are technically and often sonically superior to units that are switchable from stereo to bridged monos. According to Clayton and McCormack the dedicated internal bridging and large power supplies make the difference. Opinions are appreciated.

I’m not familiar with the internal design of the amps that have been mentioned, but I would expect that one significant difference between a bridgeable stereo amp and an amp that is internally bridged and is mono only is that in the former case two signal paths that are almost entirely separate would exist when the amp is operated in bridged mode, while that would not necessarily be the case in the latter situation, where the output stages may be mainly what is separate. Which means that it may be less challenging in the latter case to obtain an optimal match between the characteristics of the signal provided to the amp’s + output terminal and the signal provided to the amp’s – output terminal. And similarly that may be less of a challenge in the case of a “fully balanced” amplifier, which like a bridged amp provides a full amplitude signal on both its – and + output terminals. Fully balanced amps are often (although not always) designed with signal paths comprised of differential stages, and that approach may very conceivably also mitigate the challenge of keeping the two outputs matched, compared to situations in which the + and – output terminals are driven via separate signal paths.

In any event, I think the others who responded have hit upon the key points. As is usual in audio there are a multitude of approaches that can be chosen to meet a given set of requirements, each involving innumerable tradeoffs. And a designer will have his or her own preferences among those approaches based on the particular individual’s background, knowledge, experience, and the approach he or she feels most familiar and comfortable with. And what usually counts most is how well whatever approach is chosen is implemented.

Also, as alluded to earlier, bridging tends to have negative connotations in some quarters as a result of the fact that stereo amps which can be bridged usually sound worse when bridged than when operated in stereo mode. The main reason being that in bridged mode such an amp will see a load impedance equal to the speaker impedance divided by two. But I would not extrapolate from that any expectation of the sonics that would be provided by a monoblock amp that is internally bridged, and which can only be operated in that manner, and which has been robustly designed with the need to drive low impedances presumably front and center in the mind of the designer rather than being some approximation of an afterthought.

BTW, the reason a bridged amp sees half the speaker’s impedance is that from the perspective of each of the two bridged outputs a given output voltage will result in twice as much current being drawn from it as would be drawn from that output if it were operating by itself. If operating by itself the voltage appearing across the load would be the difference between the voltage it is outputting and the amp’s circuit ground, which the circuitry sees as zero volts. In a bridged configuration the voltage appearing across the load would be twice that amount, since the other side of the load is being driven with an equal and opposite voltage rather than being connected to ground. Twice the voltage across a given load impedance means twice the current. And since per Ohm’s Law resistance equals voltage divided by current, if a given output voltage results in twice as much current being drawn it appears from the perspective of whatever is providing that voltage that the load impedance has been halved.

Best regards,


Many thanks for your comprehensive and  understandable post....very helpful indeed!
Thanks to all respondents....I must remember that dedicated Monoblocks designed as internally bridged, with potent power supplies and fully balanced, like Clayton M300s, may not suffer the co͏mpromises of bridgeable stereo amplifiers. 
I am now aware that some of the finest amplifiers around, including the Constellation Reference series, and Naim Reference series, are also balanced bridged designs. Even Atma-Sphere circlotron circuits are a variant of balanced bridged design. Great designers take different roads to audio nirvana. 
@almarg As always, thanks so much for your teaching. Now I finally understand why the first watt f4 manual is written the way it is. 

"As a stereo amplifier with single-ended inputs and outputs, it will deliver up to 25 watts into 8 ohms with a damping factor of 40. It will do 50 watts into 4 ohms, and as a mono-block amplifier with parallel inputs and outputs, it will do about 100 watts into 2 ohms. As a mono-block amplifier with balanced inputs and outputs the power output rating is 100 watts into 8 ohms at 1%."

I've built one and am in the process of building a second and couldn't figure out why Nelson kept changing the impedance in his description...
Flemming was expressing his disdain for Monoblocks that are actually 2 internally bridged amplifiers

Flemming is right, and these are these are the reasons for not bridging amps.

  I’ve said it before in other threads here it is again

"Nice amp, if you want to hear them at their best, don’t bridge "if there’s no need to", as all you’ll gain is watts, everything else takes a hit when you bridge amps.

More watts.

Worse damping factor
Higher output impedance (has relevance to damping factor)
Lower stability (especially into low impedance’s)
Current ability is reduced (especially into low impedance’s)
Higher distortion.

And if you have two of them better to run them in stereo mode and vertically bi-amped, instead of bridging (mono’ing)."

Cheers George
First of all Rasmussen is not an engineer. I believe his degree is in graphic arts. He is a very rich old audiophile who had nothing else do do with his money. Wonderful, any way great audio equipment comes to life is fine by me.
Although I am ace with a soldering iron my knowledge of electronic engineering is basic at best. I only know what I hear. With the exception of the Benchmark AHB2 I have not heard a bridged amp sound as powerful as a dedicated mono amp of similar power. The Benchmark is an enigma. This is on ESLs , planars and subwoofers. 
Hi George:

Thank you again for your ‘input’.  I’m just wondering why designers/engineers from Constellation, Naim, Clayton, McCormack and other fine manufacturers, who obviously are aware of these compromises, choose to use balanced bridged designs in their reference products. Do you believe their decision is based partly on cost, generating maximal wattage without the robust power supplies and output stages needed to generate 200 wpc in a non bridged, high current, class A design?  Interesting...thanks again!
It's all out there to search, Al has also given some good info on it as well, above. If all you need is watt's then bridging is the way to go. 

Cheers George
I own both dedicated mono design amps as well as 2 channel amps ive bridged into a single channel, i even have a pair of monoblocks whos design is a 2 channel amp that's internally bridged at the factory and sold in singles as a mono amp ,ive found no design to be superior , for my ears much depends on synergy between amp & preamp as well as proper impedance matching monoblocks to the load a speaker presents at its lowest part of the curve ,case in point ,on my 8 ohm speakers my best sounding monoblocks are a pair of Mcintosh Mc-252s bridged for 500 watt @8 ohm amps when compared to my 1200 8ohm watt factory designed Mcintosh Mc-1201 monoblocks , with 4 ohm loads my factory bridged Emotiva XPR-1 monoblocks blow the pants off of the Mc-252s i run in mono as well as the purpose built Mc-1201s & Jeff Rowland true monoblocks , with my maggies my class D true monoblock Jeff Rowland amps sound better than my other amps, even better than the 2 channel amps i cannot bridge , i do believe the guy who wrote the article has went a bit off the deep end .
Hi bigjoe.

I agree with you. Although there are some technical/electrical drawbacks to bridging, if implemented well it can sound as great as any non bridged design. Years back, reviewer Peter Montcrief named his two best sounding solid state amplifiers in the world, the McCormack DNA-500 and the Clayton Audio S-2000, both just happen to be dedicated balanced bridged design Monoblocks.... expert implementation seems to be most critical. Thanks
For those interested:

Just one reviewers opinion.
Audiobrian, anybody who thinks those are the best sounding SS amps out there needs to have their Haldol doses increased. Not that they are bad but certainly not the best. Find me a bridged amp that sounds as good as a Pass 600.8 or Parasound JC-1 not to mention the Gryphon and Soulution Mono amps. Or even the Manley Neo amps or any of the Atma-sphere mono amps. Can’t do it. Not even the giant killer AHB2 can do it. 
Sh-t I am agreeing with George. I am going to have to increase my Haldol dose..
Everyone’s tastes and ears are different, of course. Mr Moncrieff may have, indeed, needed additional haloperidol in 2004, I believe, when that volume of IAR was published. Just found it interesting that his choices for the two most natural SS amplifiers (at that time) were bridged designs....proves nothing....just his opinion without mention or possibly knowledge of circuit design. My personal SS favorites for natural sound are the Gryphon Antileon Evo, GamuT M250i and Clayton S-2000, two non bridged and one dedicated bridged design, two class A and one class AB, and two bipolar and one mosfet design....lots of flavors and excellent designers.